FOOD-COLORING SUBSTANCES. 19 
the residue dissolved in water, and the constituent colors identified 
in portions of this solution. A portion of the slightly acidified solu- 
tion may be warmed, and a few decigrams of urea and then one or 
two drops of sodium nitrite solution added. Indigo Carmine is con- 
verted into the pale yellow Isatinsulphonate, while the other dyes 
are but little affected. The Isatin compound is not ordinarily pres- 
ent in sufficient concentration to tint the solution, but it differs from 
Tartrazin also in being much less readily extracted by amyl alcohol 
from strong acid solution (less than one-half from four-normal acid). 
Amaranth is much more quickly attacked by most reducing agents 
than Tartrazin, and may be destroyed in mixtures containing Tar- 
trazin by treating the neutral or faintly acid solution at room tem- 
perature with sodium hydrosulphite. (In the presence of sodium 
carbonate the reduction of Tartrazin takes place still more slowly.) 
The reagent should be added very carefully, either in dilute solution 
or as the powder, sufficient time being allowed after each addition for 
the reduction to take place. When the color shows that the Ama- 
ranth has been completely destroyed, the mixture should be shaken 
at once with air to oxidize the slight excess of hydrosulphite before it 
can react further on the Tartrazin. To separate the Indigo Car- 
mine another portion of the neutral or faintly acid solution may be 
heated to boiling and hydrosulphite added very carefully, a few par- 
ticles at a time, until all the dyes are reduced. On shaking with air 
the Indigo Carmine is quickly re-formed. 
The fourth-normal acid solution (or the salt solution) containing 
the Ponceau and Naphthol yellow S is treated with enough hydro- 
chloric acid to make it about twice normal, and is shaken out a few 
times with washed ethyl acetate. 1 The yellow is removed from the 
combined ethyl acetate extracts by shaking out with water. It must 
always be remembered that Naphthol yellow S is almost colorless in 
strongly acid solutions, and its absence in washings, etc., must never 
be assumed until they have been made alkaline. The Ponceau 3 R 
is finally separated from the acid solution by shaking the solution with 
amyl alcohol, then washing out the dye from this extract with a few 
small portions of water. 
When (with mixtures containing Orange I) the washings of the 
ethyl acetate, which should contain only Naphthol yellow S, become 
redder with alkalies, they should be combined, made fourth-normal 
with hydrochloric acid, and the contaminating orange removed by 
extracting with amyl alcohol gasoline mixture (1:1). Or they may 
be treated with one-fifth their volume of concentrated hydrochloric 
acid and the dyes extracted by shaking once with amyl alcohol. 
From this solution the yellow may be removed by washing with 
several portions of 5 per cent salt solution. 
1 Used instead of amyl acetate as suggested by Loorais, U. S. Dept. Agr., Bur. Chera. Bui. No. 1G2, p. 57. 
